The present invention relates to an electronic compensator for a piezoelectric actuator, and more particularly to a fuel injector actuator having a plurality of sets of piezoelectric elements, at least one of which is connected to an electronic compensator circuit.
A conventional piezoelectric element is a ceramic structure whose axial length changes in the presence of an electric field created by applying a voltage across the element. In typical applications, the axial length of the element can change by, for example, approximately 0.12%. In a stacked configuration of elements, the change in the total axial length of the stack is equal to the sum of the changes in axial length of each element in the stack. As is known, applying a voltage to a piezoelectric element, or to a stack of piezoelectric elements, results in a nearly instantaneous expansion of the actuator and an instantaneous movement of any structure connected to the actuator.
It is known to use a single set of piezoelectric elements, i.e., a stack of piezoelectric elements across which a common voltage is applied, to actuate a fuel injector for an internal combustion engine. Such piezoelectric actuators precisely open and close an injector valve element for precisely metering fuel flow into a combustion chamber.
The thermal and pressure effects present in the piezoelectrically actuated injector""s operating environment can cause dimensional changes within the injector. These dimensional changes result in a change to the injector""s stroke, causing an unstable shift in its flow characteristics. To compensate for the dimensional changes, it is known to fabricate injectors from exotic materials, which exhibit low thermal expansion. In addition, it is also known to calibrate injector strokes to anticipate elongation of the valve body. However, these methods are costly and inefficient.
Advantages of the claimed invention include increasing the stroke of the piezoelectric actuator, compensating for thermal expansion in different operating condition, and compensating for mechanical deformation under different fuel pressures and assembly stresses.
The present invention provides a fuel injector that comprises a tube assembly having a longitudinal axis extending between a first end and a second end; a seat secured at the second end of the tube assembly, the seat defining an opening; a stem assembly including a cap movable with respect to the tube assembly and a stem movable with respect to the seat, the stem moving between a first position wherein the stem contiguously engages the seat such that fuel flow through the opening is prevented and a second position wherein the stem is spaced from the seat such that fuel flow through the opening is permitted; a gap between the cap and the stem in the first position, the gap promoting sealing between the seat and the stem in the first position and being eliminated in the second position of the stem; a first set of piezoelectric elements contiguously engaging the cap, the first set of piezoelectric elements moving the cap in response to a first electric field; and a second set of piezoelectric elements moving the first set of piezoelectric elements in response to a second electric field.
The present invention also provides a fuel injection system that comprises a fuel injector and a control circuit. The fuel injector includes a tube assembly having a longitudinal axis extending between a first end and a second end; a seat secured at the second end of the tube assembly, the seat defining an opening; a stem assembly including a cap movable with respect to the tube assembly and a stem movable with respect to the seat, the stem moving between a first position wherein the stem contiguously engages the seat such that fuel flow through the opening is prevented and a second position wherein the stem is spaced from the seat such that fuel flow through the opening is permitted; a gap between the cap and the stem in the first position, the gap promoting sealing between the seat and the stem in the first position and being eliminated in the second position of the stem; a first set of piezoelectric elements moving the stem assembly in response to a first electric field; and a second set of piezoelectric elements moving the first set of piezoelectric elements in response to a second electric field. The control circuit includes a first driver supplying a first electrical signal generating the first electric field; a second driver supplying a second electrical signal generating the second electric field; a sensor measuring the gap and providing an output signal proportional to gap size; and a controller comparing the output signal to a reference signal and adjusting the second electrical signal in response to changes in the gap size.
The present invention also provides a method of compensating a fuel injector for thermal expansion and mechanical deformation. The fuel injector includes a tube assembly having a longitudinal axis extending between a first end and a second end, a seat secured at the second end of the tube assembly and defining an opening, a stem assembly including a cap movable with respect to the tube assembly and a stem movable with respect to the seat, the stem moving between a first position wherein the stem contiguously engages the seat such that fuel flow through the opening is prevented and a second position wherein the stem is spaced from the seat such that fuel flow through the opening is permitted, a gap between the cap and the stem in the first position, a first set of piezoelectric elements moving the stem assembly in response to a first electric field, and a second set of piezoelectric elements moving the first set of piezoelectric elements in response to a second electric field. The method comprises generating an output signal that is proportional to at least one of thermal expansion and mechanical deformation in at least one of the tube and stem assemblies; comparing the output signal with a reference signal; and adjusting the second electric field in response to variations between the output signal and the reference signal.